1. Field of the Invention
The present invention relates to automatic, exposure control devices for use in photographic printers, and more particularly, to such devices which determine exposures based on the characteristics of the photographic originals which are received for printing.
2. Description Relative to the Prior Art
It is well known to examine the characteristics, particularly the optical density characteristics, of a photographic original to determine exposure time to be used in the printing of that original. Most automatic exposure controllers for photographic printers utilize this basic examination approach in exposure determination. Such controllers typically compare the characteristics (e.g. brightness and tone balance) of the original against empirically determined "norms" in selecting exposures for printing. One shortcoming of this type of exposure control is that printing errors tend to occur in situations where a photographer has purposely departed from picture-taking norms in producing a particular original.
For each printing error, a "remake" (reprinting) is performed which requires individualized handling of the original which was unsatisfactorily copied and, attendantly, individualized handling of the customer order to which the reprinted copy belongs. Because of this special handling, each "remake" entails a tremendous increase in cost in comparison with a print routinely produced by the automatic printer.
A number of automatic printing techniques have been developed to reduce "remake" occurrences, and thus keep costs down. In one approach only a portion of the original is examined for optical density; the examined portion being selected to have a high likelihood of including the principal subject. In some of the more sophisticated techniques, comparisons are made of densities measured for two or more portions of the original (see U.S. Pat. Nos. 3,677,641 and 3,708,676).
The idea underlying these automated efforts to reduce "remake" occurrences is typically one of extracting from the original that information which is most effective for exposure control purposes. Or, considered another way, a characteristic is sought, by limiting and refining the examination of the original, which is more universally effective as a basis for exposure control.
The refined or specialized examination techniques have, to some degree, proven successful in reducing printing errors. There remains, nevertheless, a significant population of originals for which present exposure control techniques tend to produce overcorrected exposures with the consequence of removing desired photographic effects. For the most part that population includes originals which are purposely unusual in subject matter, subject placement, or tone balance. With such originals it is difficult, and frequently impossible, to extract, based on image-related characteristics, sufficient information to indicate satisfactory printing exposures.